1. Field of the Invention
This invention relates generally to the field of power window systems, and more particularly to devices and methods for maintaining a constant rate of speed at which an electric or other motor raises and lowers an automobile window.
2. Description of the Related Art
In the field of luxury automobiles, a customer's perception of quality is largely based on an automobile's level of refinement. One aspect of refinement centers on whether the operating speed of an automobile's power windows is smooth and consistent when the windows are raised and lowered. A problem common to all automobile power window systems is that the load on a window motor changes depending on whether the motor is pushing the window up or pulling it down. The force of gravity causes the load on the motor to be greater during upward travel than it is during downward travel. If this load inequality is not compensated for, the window will descend at a faster rate than it is able to ascend. This change in speed is much more noticeable with large, heavy pieces of glass such as those used in luxury vehicles.
A DC brush motor, which is commonly used to raise and lower automobile windows, converts electrical energy to mechanical energy by creating a magnetic field that pushes or pulls against permanent magnets on the motor case. The force on a current carrying conductor in a magnetic field is the product of the magnetic field strength and the current in the conductor. This relationship is described by the equation F=iL×B, where F is force, i is the magnitude of the current in the conductor, L is the direction current is traveling in the conductor, and B is magnetic field strength. The standard solution for overcoming the problem of unequal rates of power window ascent and descent has been to use pulse width modulation (PWM) to control the amount of current in a window motor's armature windings. By using PWM to vary the amount of voltage applied to a power window motor, the current flowing through the conductor may be varied and the speed-torque curve of the motor can be shifted up or down. Thus, by increasing the voltage applied to the motor during the upward travel of the window, and decreasing the voltage during the downward travel, the window's rates of ascent and descent can be matched.
It is desirable to have an alternative, mechanical solution to the problem of window motor load variation that does not require the additional electrical components necessary for employing PWM.